1. Field
The present disclosure relates to a tire having a radial carcass reinforcement and more particularly to a tire intended to equip vehicles carrying heavy loads and running at sustained speed, such as, for example, lorries, tractors, trailers or buses.
2. Description of Related Art
Generally, in tires of heavy-duty type, the carcass reinforcement is anchored on either side in the region of the bead and is surmounted radially by a crown reinforcement composed of at least two superimposed layers formed of threads or cords which are parallel in each layer and crossed from one layer to the next, forming angles of between 10° and 45° with the circumferential direction. The said working layers, forming the working reinforcement, can also be covered with at least one “protective” layer formed of reinforcing elements which are advantageously metallic and extensible, referred to as elastic. It can also comprise a layer of metal threads or cords having a low extensibility forming, with the circumferential direction, an angle of between 45° and 90°, this “triangulation” ply being radially located between the carcass reinforcement and the first “working” crown ply, which are formed of parallel threads or cords exhibiting angles at most equal to 45° in absolute value. The triangulation ply forms, with at least the said working ply, a triangulated reinforcement which exhibits, under the various stresses to which it is subjected, few deformations, the triangulation ply having the essential role of absorbing the transverse compressive loads to which all the reinforcing elements in the region of the crown of the tire are subjected.
In the case of tires for “heavy-duty” vehicles, just one protective layer is usually present and its protective elements are, in the majority of cases, oriented in the same direction and with the same angle in absolute value as those of the reinforcing elements of the radially outermost and thus radially adjacent working layer. In the case of construction plant tires intended for running on more or less undulating ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the following layer and the reinforcing elements of the radially internal protective layer being crossed with the inextensible reinforcing elements of the radially external working layer adjacent to the said radially internal protective layer.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction which intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis around which it revolves in normal use.
A radial or meridian plane is a plane which contains the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
Some current tires, referred to as “road” tires, are intended to run at high speed and over increasingly long journeys, as a result of the improvement in the road network and of the growth of the motorway network throughout the world. The combined conditions under which such a tire is called upon to run without any doubt makes possible an increase in the number of miles travelled, the wear on the tire being reduced; on the other hand, the endurance of the tire is detrimentally affected. In order to allow one, indeed even two, retreadings of such tires in order to lengthen their lifetime, it is necessary to retain a structure and in particular a carcass reinforcement having endurance properties sufficient to withstand the said retreadings.
Prolonged running under particularly severe conditions of the tires thus constructed effectively results in limits in terms of endurance of these tires.
The elements of the carcass reinforcement are in particular subjected to bending and compressive stresses during running which adversely affect their endurance. Specifically, the cords which form the reinforcing elements of the carcass layers are subjected to high stresses during the running of the tires, in particular to repeated bending actions or variations in curvature, resulting in rubbing actions at the threads and thus in wear, and also in fatigue; this phenomenon is described as “fatigue-fretting”.
In order to perform their role of strengthening the carcass reinforcement of the tire, the said cords first of all have to exhibit good flexibility and a high flexural endurance, which implies in particular that their threads exhibit a relatively small diameter, preferably of less than 0.28 mm, more preferably of less than 0.25 mm, generally smaller than that of the threads used in conventional cords for the crown reinforcements of tires.
The cords of the carcass reinforcement are also subject to “fatigue-corrosion” phenomena due to the very nature of the cords, which favor the passage, indeed even drain, corrosive agents, such as oxygen and moisture. This is because the air or the water which penetrate into the tire, for example when damaged by a cut or more simply as the result of the permeability, even low, of the internal surface of the tire, can be conveyed by the channels formed within the cords from the very fact of their structure.
All these fatigue phenomena, which are grouped together generally under the generic term of “fatigue-fretting-corrosion”, are the cause of a progressive deterioration in the mechanical properties of the cords and can affect, for the most severe running conditions, the lifetime of the cords.
In order to improve the endurance of these cords of the carcass reinforcement, it is known in particular to increase the thickness of the rubber layer which forms the internal wall of the cavity of the tire in order to limit as much as possible the permeability of the said layer. This layer is usually partly composed of butyl, so as to increase the leaktightness of the tire. This type of material exhibits the disadvantage of increasing the cost of the tire.
It is also known to modify the construction of the said cords in order in particular to increase their penetrability by the rubber and thus to limit, indeed even eliminate, the passage of oxidizing agents via the channels formed within the cords. Tires thus produced have demonstrated problems of appearance of air pockets during the manufacture of the to tire.
This is because the various stages of manufacture result in the formation of occluded air pockets. In the case of tires comprising a carcass reinforcement formed of cords, the structure of which forms channels which can convey air, these air pockets disappear as a result of the diffusion of the air into the materials, in particular through the said channels existing within the cords. In the case of tires comprising a carcass reinforcement formed of cords, the structure of which is strongly penetrated by the rubber, these air pockets remain on conclusion of the manufacturing stages. There appears solely a displacement of these air pockets during the stage of curing the tire, the pockets being displaced towards regions where a low pressure is exerted. This displacement of the air takes place along the carcass reinforcement following passages existing between the reinforcing elements, the layers of rubber mixture covering the reinforcing elements forming reinforcing regions parallel to the reinforcing elements before the stage of curing the tire. These reinforcing regions thus allow the air to be slightly displaced as a function of the pressure which is exerted on the regions where the air pockets occur. The pressure or the variations in pressure occur in particular during the stage of curing the tire or else during the shaping stage, if this exists.
The appearance of these air pockets is generally totally unacceptable according to their location and can require the scrapping of the tires, it being possible for the air pockets to become regions of weakness of the tire. The manufacturing costs then become unacceptable due to the simple fact of the low production outputs.
Furthermore, solutions consisting in deploying associated textile threads on at least one layer of the carcass reinforcement have already been provided. Such threads can make it possible to drain the air occluded during the manufacture of the tire and thus results in a greater productivity than that mentioned above and thus in more advantageous manufacturing costs. Such solutions are, for example, described in Patent Applications WO10/072,463 and WO10/072,464.
However, it turns out that, under particularly severe running conditions, in particular in terms of load and temperature, tires thus produced exhibit a reduced performance in terms of endurance in comparison with that of tires not comprising such associated textile threads on at least one layer of the carcass reinforcement.